Relative and Absolute Dating PDF

# Relative and Absolute Dating ## Key Concepts - **Relative Dating:** - Placing of events in the order in which they occurred without any relationship to the actual time during which any one event occurred. - The process of determining if one rock or geologic event is older or younger than...

# Relative and Absolute Dating ## Key Concepts - **Relative Dating:** - Placing of events in the order in which they occurred without any relationship to the actual time during which any one event occurred. - The process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages. - Used words like "older" or "younger" in describing the age of the rocks. ## Principles of Relative Dating 1. **Principle of Original Horizontality:** Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally. This allows us to infer that something must have happened to the rocks to make them tilted. This includes mountain building events, earthquakes, and faulting. - At the time of deposition, sedimentary layers are horizontal. - If they are no longer horizontal, they have undergone some type of deformation. - The two most common type of deformation are folding and faulting. 2. **Principle of Lateral Continuity:** Sediments are deposited laterally in continuous layers. Therefore if horizontal strata are dissected by erosional features, such as a valley, they can be interpreted to be the same strata as shown in the diagram below. 3. **Law of Superposition:** In an otherwise undisturbed sequence of sedimentary strata, or rock layers, the layers on the bottom are the oldest and layers above them are younger. - **Law of Superposition** → younger rocks are on top, older rocks are on bottom. 4. **Principle of Cross-Cutting Relationship:** Any geologic feature that crosscuts or modifies another feature must be younger than the rocks it cuts through. The cross-cutting feature is the younger feature because there must be something previously there to cross-cut. Cross-cutting features can include folds, faults, and igneous intrusions. They can also include events like metamorphism. - In the diagram below, the igneous dike D must be younger than fault A and igneous intrusion B, because it cuts across these (and other) features. 5. **Principle of Inclusions:** The principle of inclusions states that inclusions found in other rocks (or formations) must be older than the rock that contain them. An inclusion is any material trapped inside a mineral as it forms according to mineralogists. - Intrusion = older - Rock = younger 6. **Principle of Baked Contacts:** Magma will heat/metamorphose or "bake" the rocks it comes in contact with. Therefore if the rocks surrounding an igneous rock have a "baked contact", they must have been present before the magma cooled. 7. **Principle of Unconformity:** is a surface of non-deposition or erosion. Accumulation of sediments continues until the supply of sediments is cut off or if the area is subject to uplift and erosion. Uplift to the surface (e.g. from the bottom of the ocean to the surface) exposes rocks to the agents of weathering and erosion. - **Figure 6** illustrates the sequence of events that can lead to the formation of an angular unconformity. - The horizontally layered sedimentary unit on top is separated from the underlying folded rocks by an unconformity. - Rocks above an unconformity are younger than the rocks below. - **Types of unconformities** 1. **Angular Unconformity** - attitude of beds above and below the surface of erosion or unconformity are not the same (beds are not parallel to each other). 2. **Nonconformity** - the layer below the erosional surface is either a metamorphic rock or an igneous rock. The layer above the erosional surface is a sedimentary rock. 3. **Disconformity** - sedimentary rock strata above and below the surface of erosion are parallel to each other. 4. **Paraconformity** - strata or beds are parallel to each other. There is no discernableerosional surface; however, there is a gap in the ages between the rock units. A paraconformity represents a period of non-deposition. ## Absolute Dating - Sometimes called **numerical dating**, to give rocks an actual date, or date range, in number of years. - This is different to relative dating, which only puts geological events in time order. - Determines how much time has passed since rocks formed by measuring the radioactive decay of isotopes or the effects of radiation on the crystal structure of minerals. - Used numbers (in millions of years, mya). ## Methods of Absolute Dating 1. **Carbon-14 dating:** - A method that provides objective age estimates for carbon-based materials that originated from living organisms. - Also called **radiocarbon dating**, method of age determination that depends upon the decay to nitrogen of radiocarbon (carbon-14). - Used by the Scientists to measure the radiocarbon in the fossil to determine its age. - Used to find the age of once living materials between 100 and 50,000 years old. 2. **Potassium-Argon dating:** - Method of determining the time of origin of rocks by measuring the ratio of radioactive Argon to radioactive Potassium in the rock. - This dating method is based upon the decay of radioactive Potassium-40 to radioactive Argon-40 in minerals and rocks; Potassium-40 also decays to Calcium-40. - Used by the Scientists to measure the amount of Argon in the rock to determine its age. - Used to date rocks from 100,000 years to over a billion years old. 3. **Uranium-Lead Dating:** - Is a method that uses the radioactive decay of uranium (U) isotopes (238U, 235U, and also in this entry 232Th) into stable isotopes of lead (Pb) (206Pb, 207Pb, and 208Pb, respectively). - Is usually performed on zircon crystals. When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. If any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium.